The present invention relates to the field of Fourier transform optical correlators used for image recognition.
Optical image correlation has been applied as a pattern recognition technique for some time. See for example, A. Vander Lugt, IEEE Trans. Inf. Theory II-10, 139(1964). Although many optical correlation systems use film or etched chrome on glass plates as spatial filters, more recent implementations have used spatial light modulators (SLMs). The introduction of SLMs have made possible adaptive correlator systems which can process hundreds or even thousands of filters per second under real-time control of the system operator. At the same time, it has been shown that the most important part of the filtering operation is that done on the phase of the Fourier transform because of the large amount of image information carried with the phase. See J. L. Horner and P. D. Gianino, Appl. Opt. 23, 812 (1984).
The combination of phase-only filtering and the use of SLMs as spatial filters carries an attendant problem of phase distortion in the input image and in the filter, since ideal phase-only filtering assumes a pure intensity image with no phase distortion and a pure phase filter with no phase distortion; since the optical system uses coherent light, phase as well as amplitude must be considered at every point in the system. Previous researchers have presented methods of eliminating, or taking advantage of, the phase distortions present in the light modulators serving as the input image or the filter device. See U.S. Pat. No. 4,826,285 (1989) issued to J. L. Horner; and R. D. Juday, S. E. Monroe Jr., and D. A. Gregory, Proc. SPIE 826, 149(1987).